In the most sobering study of extinction yet, a team of Bay Area scientists says that animal species are disappearing at an accelerating rate — portending the sixth mass extinction in the 4.5-billion-year history of the Earth.

“We are entering a mass extinction equivalent to what happened to the dinosaurs” unless conservation efforts are intensified, said UC Berkeley paleontologist Anthony D. Barnosky and an author of the report, which was published Friday.

If the trend continues, “within two human lifetimes we are in danger of losing three of four species on Earth,” he said.

The 21st century may mark the end of the line for animals confined to small and shrinking habitats, such as the Yangtze River dolphin or the African black rhinoceros. And, Barnosky said, their loss may be followed by icons such as elephants and tigers.

California’s state mammal, the grizzly bear, is among species at risk of extinction by the end of the century.

Also vulnerable are California’s state tree, the redwood; the state reptile, the desert tortoise; the state amphibian, the red-legged frog; and the state mammal, the grizzly bear, he said.

“They are all moving to same status as the state fossil, the saber-tooth cat,” Barnosky said.

Grizzlies were already extinct in California three decades before the Legislature named it the state animal in 1953. But more than 1,500 still survive in states such as Idaho, Montana and Wyoming.

Based on fossil records, the expected rate of extinction of vertebrate species, without human activity, is 2 per 100 years per 10,000 species. At this rate, nine species would have been expected to go extinct since 1900.

But the actual toll has been far greater, the team found. At least 198 vertebrate species have been lost since 1900: 35 mammals, 57 birds, 8 reptiles, 32 amphibians and 66 fishes. When the category is broadened to include species that are possibly extinct or extinct in the wild, the total rises to 477.

A continued trajectory is “like going into the world’s most famous museum — say the Louvre in Paris — and slashing with a razor blade three out of every four paintings,” Barnosky said. “In one century, we’re destroying works of art that evolved over millions of years.”

There is broad consensus among scientists that extinction rates have reached their highest levels since dinosaurs died out 66 million years ago.

More recently, Duke University biologist Stuart Pimm and others have asserted that this represents the beginning of Earth’s sixth mass extinction — echoing the five great extinctions over 4.5 billion years, each depleting 75 percent or more of all species.

But that conclusion has been criticized for using assumptions that overestimate the severity of the extinction crisis.

The new findings, published in the journal Science Advances by UC Berkeley’s Barnosky and Stanford University biologists Paul Ehrlich and Gerardo Ceballos, support the dire forecasts, even though they used more modest rates of extinction.

The study shows “without any significant doubt that we are now entering the sixth great mass extinction event,” said Ehrlich, professor of population studies and a fellow at the Stanford Woods Institute for the Environment.

The work was divided between the Berkeley and Stanford labs. At Berkeley, Barnosky focused on fossil records of vertebrates, primarily mammals, which have the most reliable ancestral data. At Stanford, Erhlich and Ceballos, a Mexican conservationist, compared the rate of natural extinction before humans — 2 per 10,000 species per 100 years — with current extinction rates, derived from data from the 2014 International Union of Conservation of Nature’s “Red List.”

Past mass extinctions were caused by massive volcanic eruptions and an asteroid falling from the sky.

Now, humans are delivering the coup de grâce, according to the team. We’re emitting carbon that drives climate change and ocean acidification. We’re also cutting down forests, introducing invasive species and releasing poisons into fragile ecosystems.

If Earth’s population — now 7.13 billion — stops at 9 to 10 billion by the year 2050, other species stand a chance, Barnosky said.

But if it shoots up to 16 billion, he said, “it is probably not going to work for keeping other species on Earth.”

While it’s not too late to slow the loss of species, the window of opportunity is rapidly closing, they said. Already, the specter of extinction hangs over about 41 percent of all amphibian species and 26 percent of all mammals.

“I don’t want to be the generation that wiped out all these species,” Barnosky said.

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